Write a program that displays guitar chords specified by the user, with the notes marked as points on each guitar string.
A visit to here might help you with some of the details of guitar chords.
Can be used as an early assignment to exercise use of loops, arrays and modular arithmetic, and perhaps equally important, that it's necessary to delve into some domain-specific knowledge in order to produce a useful software system — it's not all pure programming!
Can also be used as a later assignment to separate logic from user interface, i.e. object-oriented design.
When I gave this assignment to first year students middle 1990's some of them produced astounding graphical user interfaces implemented from scratch using Borland Pascal in DOS. Some others complained that they couldn't possibly do it since they didn't know anything about guitars. The latter group needed a lot of coaching in the beginning.